1. Field
The present invention relates generally to digital video, and more specifically to video compression.
2. Background
Due to the explosive growth and great success of the Internet and wireless communication, as well as increasing demand for multimedia services, streaming media over the Internet and mobile/wireless channels has drawn tremendous attention. In heterogeneous Internet Protocol (IP) networks, video is provided by a server and can be streamed by one or more clients. Wired connections include dial-up, integrated services digital network (ISDN), cable, digital subscriber line protocols (collectively referred to as XDSL), fiber, local area networks (LAN), wide area networks (WAN) and others. The transmission mode can be either uni-cast or multi-cast.
Similar to the heterogeneous IP network is mobile/wireless communication. Transport of multimedia content over mobile/wireless channels is very challenging because these channels are often severely impaired due to multi-path fading, shadowing, inter-symbol interference, and noise disturbances. Some other reasons such as mobility and competing traffic also cause bandwidth variations and loss. The channel noise and the number of users being served determine the time-varying property of channel environments.
Digital video is typically compressed for efficient storage and/or transmission. Many standards of video compression exist.
One problem common to video compression is a compromise between bandwidth (bits per second) and visual quality. Various measures, such as peak signal-to-noise ratio (PSNR), can be used to assess visual quality. It will be understood that with a constant frame rate, the bits used for encoding frames of video will be directly proportional to the bit rate or bandwidth for the video, and that these terms (bits and bandwidth), while technically are not the same, are often used interchangeably in the art and the correct interpretation can be determined from context.
The bandwidth needed for relatively good visual quality will vary with the complexity of the video being encoded. For example, a relatively static shot, such as a shot of a newscaster, can be encoded at relatively high visual quality with relatively low bandwidth. By contrast, a relatively dynamic shot, such as a shot panning athletes in a sporting event, can consume a relatively large amount of bandwidth for the same visual quality.
To achieve a relatively constant quality, it is desirable to use a technique known as variable bit rate (VBR) to vary the number of bits available to encode the frames. However, these VBR techniques are generally not applicable in the context of transmitted video content. Digital video content can be transmitted via a variety of mediums, such as via optical networks, wired networks, wireless networks, satellites, and the like. When broadcasting, these communication mediums are typically band limited. Accordingly, constant bit rate (CBR) techniques are typically found in the transmission or broadcast environment.
A problem with the use of constant bit rate (CBR) techniques is that the visual quality will vary according to the complexity of the video being encoded. When a shot is relatively static, such as a shot of a broadcaster, then more bits than needed for a given quality level will be consumed or “wasted.” When a shot is relatively dynamic, such as in sports, then the quality can suffer with CBR. When the visual quality is compromised, visual artifacts can become apparent and can be observed as for example, “blockiness.”
There is therefore a need in the art for encoding techniques which can combine the favorable attributes of relatively constant bit rate for the transmission medium, and relatively constant visual quality for the viewer's enjoyment.